interlibrary loan is borrowing a book/written material from other libraries through your library. go to your university's library website and look for it. like samikshya said, borrow books from library if you can't afford to buy them. i've done that for a few classes in the past. but if I buy books, i usually buy it from other students who have already taken the class (my university has a website where students can sell their used books to other students taking the same class) and when i'm done with the class, i sell it to others (for the same price +/- 5-10 bucks) who take the class after me. if you university has something like that, use it! saves you tons!
also, not legal but i've done it in the past - search for online pdf copies if you like to read on computer screen, takes time to search but you don't have to buy book.